REPORT ON THE CRIXOIDEA. 09 



protected by it, is a group of three tentacles arising from tlie radial water-vessel. These 

 groups of tentacles alternate on the opposite sides of the ambulacrum along the whole 

 length of the arm, and they accompany its lateral branches on to the pinnules. 



In the irregular genus Actinometra the mouth is not central or sub-central, but 

 excentric or even marginal. It is sometimes radial in position, as in Actinometra strota 

 and Actinometra jukesi (PI. LV. figs. 1, 2),' and sometimes interradial, as in Actino- 

 metra magnijica (PI. LVI. fig. 7) ; while the number of ambulacra diverging from the 

 l^eristome may vary, especially in the multiradiate species, from four to ten or even more. 

 In most cases, however, the large intorpalmar area which contains the central or sub- 

 central anal tube is bounded by two large aboral groove-trunks, which form a horse-shoe 

 shaped curve, and give off the ambulacra of the lateral and posterior arms. If the mouth 

 be radial these grooves are often tolerably equal and symmetrically arranged, as in 

 Actinometra strota and Actinometra jukesi (PI. LV. figs. 1, 2). But in ty|3es witli an 

 interradial mouth like Actinometra magnijica or Actinometra stelligera (PL LVI. 

 figs. 7, 8),'^ the right or western limb of the curve is much larger than its fellow. [See 

 p. 88.] The latter supplies the arms of the left lateral ray only ; while the larger right 

 limb represents the posterior ambi;lacrum combined with part or the whole of the right 

 lateral ambulacrum as well. 



In many forms of Actinometra more or fewer of the arms which arise from the 

 posterior and postero-lateral portions of the disk are in the ungrooved and non-tentacu- 

 late condition, as has been already described ; and the disk ambulacra corresponding to 

 them are less developed than those of the normal grooved arms. In fact they may be 

 altogether absent in some of the large multiradiate species such as Actinometra magnijica. 

 and Actinometra nohilis, both of which are remarkable for the presence of ungrooved 

 arms on each ray. In Actinometra magnijica for example, which has an interradial 

 mouth (PL LVI. fig. 7), not only are all the seventeen arms of the hinder ray entii-ely 

 unprovided with food-grooves, but more or fewer of the closely crowded arms on each of 

 the other rays are in the same condition, as they do not receive branches of the dividing 

 groove-trunks which supply their fellows. 



Faint lines are sometimes visible, indicating the directions which these grooves should 

 have taken, i.e., the positions of the simple water-vessels which are distributed to the 

 ungrooved arms, but give oft' no tentacular branches. This is particularly well seen in the 

 case of the posterior ray and parts of the hinder divisions of the two lateral rays of Actino- 

 metra magnijica (PL LVI. fig. 7). The primary groove-trunk, which leaves the right lower 

 corner of the peristome to supply tlie right and posterior rays, divides into two princij^al 

 branches before reaching the equator of the disk in which the anal tube is situated. 



1 The specific formula of ^c(inome«ra j«/im is— a. R. -^ — ; ;,'°j-^°^]""i I- 



2 The specific formula of Actinometra stelligera is— a. 2.2.— . 



